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Naming in the US Space Shuttle (STS) program

The shuttle is officially referred to as the Space Transportation System
(STS). Specific shuttle missions are therefore designated with the prefix
"STS." Initially, the launches were given sequential numbers indicating
order of launch, such as STS-9.

Beginning in 1984, each mission was assigned a code, such as STS-41-B,
with the first digit indicating the federal fiscal
year offset into the program (so 41-B was scheduled for FY 1984, 51-L
originally for FY 1985 and the third flight in FY 1995 would have been
named 151-C), the second digit indicating the launch site (1 was Kennedy
Space Center and 2 was Shuttle Launch Complex (SLC) 6 at Vandenberg
Air Force Base, although Vandenberg was never used), and the letter
indicating scheduling sequence. These codes were assigned when the launches
were initially scheduled and were not changed as missions were delayed
or rescheduled.

After the Challenger
disaster, a sequential numbering system was restored, with the number
according to counting from the beginning, although, unlike the initial
system, the assignment of numbers is based on the initial schedule and
may not reflect launch order. The letter indicates that the intermediate
numbering system applies, and e.g. flight STS-51
was many years after STS-51-A.

STS-300
was the designation for the Space
Shuttle rescue missions to be launched on short notice for STS-114
and STS-121.
After STS-121, the rescue flight for STS-115,
if needed, would have been STS-301.
After STS-115, the rescue mission designations changed again. They are
now based on the corresponding regular mission that would be replaced should
the rescue mission be needed. For example, the STS-116
rescue mission was branded STS-317,
because the normal mission scheduled after STS-116 was STS-117.
Should the rescue mission be needed, the crew and vehicle for STS-117 would
assume the rescue mission profile and become STS-317.